1. Field of the Invention
The present invention relates to a LCD backlight inverter, in particular, which comprises an overvoltage protection circuit for controlling the overvoltage with a stable output in respect to the fluctuation of an input voltage and preventing an effect to a feedback controlling block in opening a lamp used as a backlight.
2. Description of the Prior Art
In general, an overvoltage protection circuit of a LCD backlight inverter of the prior art is constructed as shown in FIG. 1, in which the reference number 10 designates a DC/AC converter for converting voltage in the circuit and a buck switch Q3 is connected between a supply voltage Vin and the DC/AC converter 10 to determine power supply. Here, the buck switch Q3 is on/off controlled via an output signal of a switching controlling block 20, in which an overvoltage protection block 30 divides the voltage inputted into the DC/AC converter 10 by using a number of resistances and a result of dividing voltage is inputted into a detection signal judging block 40.
Here, the detection signal judging block 40 has a noninversion terminal (designated by +) connected with a reference voltage V1, and an inversion terminal (designated by xe2x88x92) where the result of dividing voltage in the overvoltage protection block 30 is inputted together with a feedback signal F/B. In other words, the switch controlling block 20 on/off controls the buck switch Q3 according to a result of comparing in the detection signal judging block 40.
In the overvoltage protection circuit of the LCD backlight inverter of the prior art shown in FIG. 1, when a load connected to the rear end or a lamp used as a LCD backlight is opened, a voltage applied to the overvoltage protection block 30 or a point a is elevated so that an output side voltage of the DC/AC converter is elevated to form an overvoltage.
In order to control this problem, a feedback control is performed so that the voltage is divided via voltage detecting resistances designated as the reference numerals R1 and R2 and the result of dividing voltage in the overvoltage protection block 30 is inputted into the inversion terminal of the detection signal judging block 40.
However, in the foregoing methods of the prior art, there have been problems that the voltage fluidly changes according to the change of the supply voltage in the control of the output side overvoltage state of the DC/AC converter 10 and a feedback F/B stage in the rear end is used together in normally operating the lamp in the rear end so that the F/B stage may be influenced according to a voltage at a point a. Therefore, it is difficult to manufacture or design the F/B stage.
The present invention is proposed to solve the foregoing problems of the prior art and it is therefore an object of the invention to provide an overvoltage protection circuit of a LCD backlight inverter, in particular, which comprises an overvoltage protection circuit for controlling an overvoltage with a stable output in respect to the fluctuation of an input voltage and interrupting an effect to a feedback controlling block in opening a lamp used as a backlight.
According to a preferred embodiment of the invention to obtain the foregoing object, it is provided a LCD backlight inverter comprising: a buck switch for determining power supply to a DC/AC converter for converting voltage, the buck switch being connected between a supply voltage input terminal and the DC/AC converter; an overvoltage protection circuit connected to an input terminal of the DC/AC converter for dividing voltage while providing a current path at the time of the initial drive; a first comparing block for comparing the dimension of a feedback signal with the dimension of a first reference voltage, wherein the feedback signal detects the dimension of the voltage according to on/off status of the backlight lamp in the rear end; a second comparing block for comparing the dimension of the voltage divided in the overvoltage protection circuit with the dimension of a second reference voltage; and a switching controlling block for receiving output signals from the first and second comparing blocks and controlling on/off status of the buck switch according to corresponding comparison signals.
According to an additional feature of the invention to obtain the foregoing object, the first comparing block includes: a first error amplifier having an inversion data input terminal for receiving the feedback signal, wherein the feedback signal detects the dimension of the voltage according to on/off status of the backlight lamp, and a noninversion data input terminal for receiving a first reference signal, to compare the feedback signal and the first reference signal; and a first condenser connected between the output terminal and the inversion data input of the first error amplifier.
According to another additional feature of the invention to obtain the foregoing object, the second comparing block includes: a comparator having an inversion data input terminal for receiving a voltage divided from the overvoltage protection circuit, and a noninversion data input terminal for receiving a second reference signal, to compare the divided voltage and the second reference signal; and a second condenser connected between the output terminal and the noninversion data input terminal of the comparator.
According to further additional feature of the invention to obtain the foregoing object, the second comparing unit includes: a second error amplifier having an inversion data input terminal for receiving a voltage divided from the overvoltage protection circuit, and a noninversion data input terminal for receiving the second reference signal, to compare the divided voltage ad the second reference signal; a third condenser connected between the output terminal and the inversion data input terminal of the second error amplifier; and a diode having a cathode terminal connected to the output terminal of the second error amplifier and an anode terminal connected to the output terminal of the first comparing block.